FIG. 1 illustrates a gas turbine engine 3. Several different hydraulic actuators, illustrated as pistons 6a-d, are used to operate components in the engine. For example, actuator 6a opens and closes a bleed door 9 which bleeds pressurized air 11 from booster stage 14. The booster 14 is a low-pressure compressor, and bleeding is sometimes necessary to match the output of the booster, at point 16, to the input requirements of the high-pressure compressor 18. Door 9 is commonly referred to as a variable by-pass valve (VBV).
A second actuator 6b operates variable stator vanes (VSV), which are shown in more detail in FIG. 2A. Varying the angle of the VSV's by rotation, indicated by circular arrow 21, allows one to control the direction of the airstream 24 which enters the compressor blades 27, thereby controlling the angle of attack of the compressor blades 27. VSV's are used to improve the performance of the compressor under acceleration.
A third actuator 6c in FIG. 1 controls a valve 30 which blows hot (or cold) air 33 upon turbine casing 36 in order to expand (or shrink) the casing 36 to thereby control the clearance 39 between turbine blades 41 and the casing 36. The air is commonly bled from the high-pressure compressor 18 as indicated. It is desirable to maintain as small a clearance 39 as possible in order to minimize leakage through the clearance. Leakage represents a loss because the leaking air imparts virtually no momentum to the turbine blades 41, and the energy in the leaking air is wasted.
A fourth actuator 6d in FIG. 1 controls a fuel valve 43 which controls the amount of fuel delivered to combustors 44.
Four types of actuators have been described, and other types are also in use in gas turbine aircraft engines. For example, there are actuators involved in the thrust reversing system, in exhaust nozzles which are variable in area, and in thrust vectoring systems used in vertical takeoff and landing (VTOL) aircraft. Further, it is foreseen that, as gas turbine technology advances, an even greater number of hydraulic actuators will come into use.
For each actuator 6a-d, a control is needed, which usually takes the form of a servovalve. The servovalve controls the flow rate and pressure of hydraulic fluid applied to the actuator, thus controlling the position of the actuator, thereby controlling, for example, the amount of fuel delivered by fuel valve 43 in FIG. 1. The use of an individual control for each actuator just described results in significant cost, weight, and mechanical complexity.